A wireless network may employ orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA). In a cellular wireless network, each cell employs a base station (designated by Node B or eNB) that communicates with user equipment (UE), such as a cell phone, a laptop, or a PDA. Base station eNB transmits reference signals or pilot signals to UE, which generates a channel estimate based on the reference signal, has impacted by interference and noise. The system bandwidth is divided into frequency-domain groups or subbands that encompass resource blocks RBs according to group size or subband size. An RB is the smallest allocation unit available in terms of frequency granularity allocated to UE by a base station scheduler module.
UE determines a channel quality indicator (CQI) for each RB or for each subband based on the channel estimation. The CQI metric is suitably a signal to interference noise ratio (SINR) after detection, the index to a supportable modulation and coding scheme, the index to a supportable code rate, a channel throughput measure, or other quality measure. UE feeds back the CQI for each subband or RB to eNB. More favorable CQI permits a higher data transfer rate of data streams by eNB to UE.
By using multiple transmit and multiple receive antennas with transmit pre-coding in a multi-input multi-output (MIMO) system, improved throughput and/or robustness are obtained. Pre-coding in a MIMO system involves determining and applying a linear or complex linear transformation for each RB to the data stream(s) allocated to the RB by an eNB scheduler prior to transmission via physical antennas. The number of independent data streams (number of spatial codewords) is termed the transmission rank. Denoting a P×R precoding matrix for each downlink RB as PM and the R independent data streams as an R-dimensional vector s, the transmitted signal via P physical antennas (P>=R) is written as: x=PM s. For a frequency division duplex FDD system where the uplink and downlink channels are not reciprocal, precoding matrices to contribute to the matrix PM are efficiently chosen at UE by indexing to a pre-determined set of matrices (pre-coding codebook). Based on the channel estimate, UE feeds back to the base station for each of its subbands or RBs, the precoding matrix index (PMI) and the CQI expected to occur when eNB uses the indexed precoding matrix to transmit data in an RB in a given subband.
A high level of operational overhead and uplink bandwidth is believed to have hitherto been involved when each of many UEs deliver feedback about many subbands to eNB. This can undesirably increase system processing delays and dissipation of power and energy which is of particular concern in mobile handset forms of UE. Accordingly, further ways of reducing the amount of communications feedback between user equipment and base station are desirable.